A vehicle body (in particular, a vehicle body frame) is required to reduce its weight for the purpose of, for example, improving the fuel consumption of a vehicle. To cope with weight reduction of a vehicle body frame, there is known a technique of thinning the vehicle body frame to reduce its weight by using a frame of a high strength.
As a weight reducing means, for example, it is known that a high-tensile steel plate (so-called high tension material) is employed as a vehicle body frame, or a frame of a high strength made by hot-stamping (that is, hot-pressing) a blank material of a steel plate using a forming die is employed as a vehicle body frame. The frame of a high strength made by hot stamping will be referred to as a high-strength frame hereinafter.
When the high-strength frame is employed as a vehicle body frame such as the center pillar, front side frame, or rear side frame of a vehicle body, the vehicle body frame can be thinned, and the weight of the vehicle body can be reduced.
The front side frame or rear frame of the vehicle body frame normally incorporates reinforcing members partially arranged to reinforce the frame or absorb an impact load (impact energy). The reinforcing members are partially arranged to generate a strength difference between predetermined portions of each frame to facilitate deformation of the frame, thereby absorbing the impact load.
However, when the reinforcing members are arranged in each frame, the weight of the front side frame or rear frame increases.
On the other hand, if a high-strength frame is employed as the front side frame or rear frame for weight reduction, each frame need not be reinforced by reinforcing members, and the weight of the vehicle body can be reduced. However, since no reinforcing member is provided in each frame, it is difficult to generate a strength difference by reinforcing members.
For this reason, each frame is hardly deformed by an input impact load, and the front side frame or rear frame can hardly absorb the input impact load.
As a measure against this, it is known that a front side frame employing a high-strength frame is formed such that its strength continuously rises from the front end to the rear side of the vehicle body (for example, see Japanese PCT National Publication No. 2012-528752).
According to the front side frame of Japanese PCT National Publication No. 2012-528752, the strength of the front portion of the front side frame is suppressed. Hence, if the vehicle collides against a hard object such as a concrete wall, it is possible to absorb the impact load by sequentially buckling and deforming the front portion of the front side frame by axial collapse.
However, if the vehicle collides against an object (for example, a vehicle) softer than a concrete wall, the front portion of the front side frame is considered to be buried in the soft colliding object. It is therefore difficult to absorb the impact load by sequentially buckling and deforming the front portion of the front side frame by axial collapse.
Additionally, if the front portion of the front side frame is buried in the colliding object, the front portion of the front side frame is supported by the colliding object. Hence, it is considered that a bending moment acts on the buried front portion of the front side frame.
The front side frame is a hard member because a high-strength frame is employed. For this reason, if a bending moment acts on the front portion of the front side frame, the front portion of the front side frame may become cracked.
It is also considered that the front side frame deforms and absorbs the impact load in the state in which the front portion of the front side frame is buried in the colliding object. However, if the front side frame deforms in the state in which the front portion of the front side frame is buried in the colliding object, the deforming portion of the front side frame can hardly be determined to a predetermined position. That is, the position where the front side frame deforms cannot be known.
Hence, since the shock absorbing performance of the front side frame is unstable, there is room for improvement from this viewpoint.